Inkjet printers are mostly known as printing devices used in printers, copying machines, or the like. Particularly, inkjet printers that employ a method utilizing heat energy as ink discharging energy and discharge ink by bubbles generated by the heat energy have recently come into general use.
An inkjet printhead, used in the above-described inkjet printers, employs an electrothermal transducer (hereinafter referred to as a heater) for generating heat energy. And in many cases, one heater is provided for one discharge orifice (nozzle).
Meanwhile, as disclosed in Japanese Patent Application Laid-Open No. 08-183179, there is a technique which enables printing in various printing modes by utilizing an inkjet printhead comprising plural heaters for one discharge orifice to vary the amount of ink discharged from each discharge orifice.
For example, one inkjet print head can realize both high-speed printing and high-quality printing by the following functions. That is, in the high-speed mode, high-speed printing with a low printing resolution is realized by increasing the amount of ink droplets discharged from respective discharge orifices so as to enlarge the size of a dot that can be printed by one ink droplet. In the high-quality mode, printing is realized at a high printing resolution by reducing the amount of ink droplets discharged from respective discharge orifices so as to reduce the size of a dot that can be printed by one ink droplet.
This compatibility of the printhead provides a great advantage in that a user can obtain a desired output image by selecting the most appropriate printing mode.
Japanese Patent Application Laid-Open No. 09-286108 discloses an inkjet printhead to meet the demands. It discloses a technique for achieving a high tonality by providing a plurality of heaters in one nozzle to change the size of a printing dot.
FIG. 27 shows an equivalent circuit of an electric circuit formed on the printhead substrate, disclosed in the aforementioned Japanese Patent Application. The circuit includes: multi-valued heaters in the ink-flowing channel which forms one nozzle; NMOS transistors 301 serving as a driving transistor for independently driving the elements 201(1), 201(2), . . . , 201(n) which serve as the multi-valued heaters; a shift register 302 configured with a CMOS transistor for processing a driving signal; a latch circuit 303 for holding data; and AND circuits 307 connected to the respective transistors 301.
The AND circuits 307 perform a logical operation on a block selection signal (Block ENB) 304 which divides the ink-flowing channel forming a nozzle into blocks, a select signal (Select) 305, data thereof, and a driving pulse signal (Heat ENB) 306, and drive the corresponding transistors 301 based on a result of the operation. Group S is formed by S(1) to S(m) so as to correspond to the number of ink-flowing channels m.
An electrode wiring 203 individually supplies electric power to one end of the elements 201(1), 201(2), . . . , 201(n) serving as the n numbers of multi-valued heaters provided in one nozzle. Each of the other ends of the multi-valued heaters is connected to a common power source 309. Furthermore, a temperature adjusting sub-heater 311, a temperature sensor 312, and a heater resistance value monitoring heater 313 are provided.
In FIG. 27, VDD denotes a logic power source, H-GND denotes a GND for the heater-driving power source 309, and L-GND denotes a GND for the logic power source VDD. The heater-driving power source 309 is connected to an end portion of all the elements 201(1) to 201(n) of the groups S(1) to S(m). The shift register 302 inputs a serial image data input signal (Idata) that corresponds to each of the groups S(1), S(2), . . . , (Sm) and a clock input signal (Clock) for driving the shift register, and outputs the image data to the latch circuit 303 as a parallel signal. To the latch circuit 303, a reset signal (Reset) and a latch signal (LTCLK) are inputted. The latch circuit 303 temporarily stores the image data inputted from the shift register 302, and outputs it to the AND circuits 307 of the respective groups S(1), S(2), . . . , S(m). The driving pulse signal (Heat ENB) 306 is inputted to the respective heaters 201(1), 201(2), . . . , 201(n) of the groups S(1) to S(m).
The select signal 305 in FIG. 27 is inputted to the input terminals 1 to n (Select 1–n) that are commonly provided to the groups S(1) to S(m). By the select signal 305, heaters subjected to heating in the respective groups S(1) to S(m) can be selected.
In FIG. 27, numeral 314 denotes a decoder. The block selection signal 304 is inputted to input terminals 1, 2 and 3 of the decoder 314. Five output terminals of the decoder 314 are connected to the AND circuits 307 of the respective groups S(1) to S(m). For instance, assuming that the number of groups S is 160 (S(1) to S(160)), i.e., the number of nozzles is 160, the first output terminal of the five output terminals is connected to AND circuits 307 of the groups S(1) to S(20) that correspond to the nozzle numbers 1 to 20. The second output terminal is connected to AND circuits 307 of the groups S(21) to S(40) that correspond to the nozzle numbers 21 to 40. The third output terminal is connected to AND circuits 307 of the groups S(41) to S(60) that correspond to the nozzle numbers 41 to 60. The fourth output terminal is connected to AND circuits 307 of the groups S(61) to S(80) that correspond to the nozzle numbers 61 to 80. The fifth output terminal is connected to AND circuits 307 of the groups S(81) to S(100) that correspond to the nozzle numbers 81 to 100. The sixth output terminal is connected to AND circuits 307 of the groups S(101) to S(120) that correspond to the nozzle numbers 101 to 120. The seventh output terminal is connected to AND circuits 307 of the groups S(121) to S(140) that correspond to the nozzle numbers 121 to 140. The eighth output terminal is connected to AND circuits 307 of the groups S(141) to S(160) that correspond to the nozzle numbers 141 to 160.
In a case where the decoder 314 is connected in the above-described manner, 8 blocks of nozzles, each connected to the same output terminal of the decoder 314, are selected as nozzles to be heated for discharging ink in accordance with the block selection signal 304, and the ink discharge timing of the 8 blocks of nozzles can be controlled.
Next, a detailed configuration of an inkjet printhead is described.
FIG. 28 is a diagrammatic cross-section showing a part of a printhead having a conventional configuration.
Numeral 901 denotes a p-type semiconductor substrate formed with monocrystal silicon. Numeral 912 denotes a p-type well area; 908, an n-type drain area; 916, an n-type electric field relaxing drain area; 907, an n-type source area; and 914, a gate electrode. The above-described components form a MIS (Metal Insulator Semiconductor)-type field effect transistor 930, which serves as a switch device using an MIS-type field effect transistor. Numeral 917 denotes a silicon oxide layer serving as a thermal storage layer and an insulating layer; 918, a tantalum nitride layer serving as a thermal resistance layer; 919, an aluminum alloy layer serving as a wiring; and 920, a silicon nitride layer serving as a protection layer. The foregoing layers constitute a printhead base 940. Numeral 950 denotes a heating portion. Ink is discharged from an ink discharge portion 960. A top plate 970 and the printhead base 940 form a liquid path 980.
Various improvements have been made on the printhead and switch device having the above-described configuration. Recently, there are increasing demands for high-speed driving, energy saving, high integration, low cost, and high performance of the product. Therefore, a plurality of MIS-type field effect transistors 930 shown in FIG. 28, serving as a switch device, are provided in the semiconductor substrate 901, and alone or a plurality of the MIS-type field effect transistors 930 are simultaneously operated to drive the electrothermal transducers connected.
However, if the conventional MIS-type field effect transistor 930 is used under a large electric current which is necessary for driving the electrothermal transducers, the p-n reverse bias junction between the drain and well cannot withstand the intense electric field, generating a leak current. Therefore, it cannot withstand the pressure required as a switch device. Furthermore, if the MIS-type field effect transistor serving as a switch device has a large resistance when it is turned on, an unnecessary current is consumed. Therefore, a current necessary for driving the electrothermal transducers cannot be obtained.
To solve the problem of the withstanding pressure, an MIS-type field effect transistor 1020 shown in FIG. 29 may be considered.
In FIG. 29, a semiconductor substrate 1001, an n-type source area 1007, an n-type drain area 1008, a gate electrode 1004, a silicon oxide layer 1017 serving as a thermal storage layer and an insulating layer, a tantalum nitride layer 141 serving as a thermal resistance layer, an aluminum alloy layer 154 serving as a wiring, a silicon nitride layer 1020 serving as a protection layer, a printhead base 152, a heating portion 1050, an ink discharge portion 153, a top plate 156, and a liquid path 155 are respectively similar to the aforementioned semiconductor substrate 901, n-type source area 907, n-type drain area 908, gate electrode 914, silicon oxide layer 917 serving as a thermal storage layer and an insulating layer, tantalum nitride layer 918 serving as a thermal resistance layer, aluminum alloy layer 919 serving as a wiring, silicon nitride layer 920 serving as a protection layer, printhead base 940, heating portion 950, ink discharge portion 960, top plate 970, and liquid path 980 shown in FIG. 28.
The configuration of the MIS-type field effect transistor shown in FIG. 29 is different from that of an ordinary transistor. In the p-type semiconductor substrate 1001, the n-type source area 1007 is surrounded by a p-type base area 1005, so that a part of the n-type well area 1002 is used as a drain. This is called a DMOS (Double diffused MOS transistor). By forming a channel within a drain as described above with the use of the n-type well area 1002, it is possible to deepen the drain that determines the withstanding pressure and to form the drain at low density, making it possible to solve the problem of withstanding pressure.
Although such a configuration as disclosed in the above-described Japanese Patent Application Laid-Open No. 09-286108 can achieve a high tonality, it requires a plurality of driving circuits, and it is necessary to provide selection signal input terminals for selecting plural heaters. Therefore, it raises a problem of an enlarged size of the substrate to be solved.
However, in a case of employing an inkjet printhead where one heater is provided for one discharge orifice, it is difficult to change the ink discharge amounts in multi-levels to be discharged from one orifice.
Furthermore, if the configuration where plural heaters are provided for one discharge orifice is adopted to change the ink discharge amounts in multi-levels, the circuit formed on the substrate of the inkjet printhead becomes complicated, because the number of heaters and driving circuits thereof becomes as many as multiple times of the number of discharge orifices, and the driving circuits for the plural heaters should be localized for each discharge orifice in layout. As a result, the cost of the printhead increases.
As described above, it is desirable to provide a printhead which enables to discharge relatively different amounts of ink with a simple structure.